This invention relates to a method of depositing thin films consisting mainly of carbon.
Carbon thin films are very suitable for making surfaces abrasion-proof, smooth, or highly thermal conductive. Such a carbon film is deposited by use of chemical vapour reaction. A plasma state is caused from a reactive gas by applying high frequency electric energy between a pair of electrodes between which the deposition space is defined. In the deposition space, a hydrocarbon gas and a carrier of hydrogen are excited into plasma gas, and a carbon film is deposited on a surface. During the deposition, positive ions such as hydrogen ions are drifted toward the surface by a self-bias voltage between the pair of electrodes and sputters the depositing film. The sputtering functions to reduce the formation of the carbon bonds based on the sp and sp.sup.2 orbitals and to increase the proportion of the carbon bonds based on the sp.sup.3 orbital, and thereby the carbon film tends to grow in diamond structure.
There are two methods for increasing the bias voltage built between the pair of electrodes. One method is to decrease the pressure of the reactive gas in the deposition space. The relative energy given to the reactive gas is increased in proportion to the decrease of the reactive gas in the reaction chamber. The molar ratio of ions to non-ions increases as the pressure decreases. The ions tend to linger about the electrodes and produce a self-bias therebetween. The other method is to increase the input power of the high frequency power and directly increase the number of positive ions.
The sputtering, however, tends to give damage to the surface to be coated, and degrades the property of the interface between the surface and the film. In addition to this, the high degree of hardness itself inherently incurs a disadvantage. Because of the hardness, when expansion or contraction takes place due, e.g. to temperature change, the interface can not bear the tension caused by the differential expansion or contraction ratio between the film and the underlying surface, and eventually the film comes off from the surface.